merge: Support inner join

[victorlin]

Context

augur merge is hardcoded to do an outer join for metadata and sequences. There have been talks about wanting to do an inner join.

Potential solution

Add a new command-line option to specify whether to do an inner or outer join, with default being outer.

Option name/value ideas:

• --join <inner|outer>
• --join-type <inner|outer>

The implementation can use SQL's INNER JOIN for metadata and seqkit common for sequences.